The thymidine kinase isozymes of normal cells and of cells infected with vaccinia virus and avian, human, simian, and porcine herpesviruses are being extensively purified by a rapid affinity chromatography method. After labelling cells with S35-methionine, the subunit polypeptides of thymidine kinase will be identified. The control of the turn-on and the turn-off of thymidine kinase polypeptide formation will then be investigated. The SV40 T antigen, presumed to be the product of the ts A gene, has been purified. Using various SV40 ts A, mutants, the biological function of the T antigen will be investigated. BIBLIOGRAPHIC REFERENCES: S. Kit and G. Kit. Analyses of deoxycytidylate deaminase molecular forms in human-mouse and monkey-mouse somatic cell hybrids. Biochemical Genetics 13: 213-226 (1975). S. Kit, W.-C. Leung, D. Trkula, and D. R. Dubbs. Characterization of nucleoside phosphotransferase and thymidine kinase activities of chick embryo cells and of chick-mouse somatic cell hybrids. Arch. Biochem. Biophys. 169: 66-76 (1975).